Embryonic stem (ES) cells are in an undifferentiated state and have an ability to develop into any type of differentiated cells in vitro in developmental processes of living organisms. It is known that the self-renewal ability and the undifferentiated state of ES cells can be maintained by using a culture medium supplemented with fetal bovine serum in the presence of feeder cells or LIF (Zandstra, P. W., et al, Biotechnol Bioeng 69, 607-17 (2000)). However, under such culture conditions currently used widely, it is difficult to remove feeder cells without fail when analyzing the differentiation of ES cells, and therefore, the effect caused by the addition of differentiation-inducing factors-cannot be analyzed correctly. Further, though ES cell lines that can be cultured without feeder cells are known, for instance, ES-D3, one of mouse ES cell lines, tends to differentiate spontaneously under the culture conditions without using feeder layer.
In addition, a serum contains fluctuating amounts of components of activin and fibroblast growth factors or unknown differentiation-inducing factors. When analyzing cell growth and differentiation of ES cells after adding various substances exogenously, these components may have an effect on results of analysis. Further, there is a risk of infection with viruses, prions and unknown factors in the use of serum, and its application to regenerative medicine involves a risk. Furthermore, though serum-free culture conditions of ES cells have been reported, ES cells may differentiate into nerves, etc., after several passages and cannot always maintain their undifferentiated state.